Metallic foils, such as aluminum foil, tin foil and the like, are useful materials in forming laminated structures. These materials may add strength to such laminates, act as light and/or vapor barriers, or merely act to give a "quality look" to packaging materials.
When forming laminated structures which include one or more metallic foil layers, it is often necessary to coat or prime the metallic foil prior to lamination. This is true, for example, when bonding aluminum foil to plastics resin film layers, such as polypropylene. Such coatings as epoxys, vinyls, polypropylene dispersions, nitrocellulose, ethyl cellulose and others are thus routinely applied to metallic foils.
During production, these coatings are applied to the metallic foils by unwinding the foil in web form from a roll, coating the foil, drying and/or curing the coating and rewinding the foil onto another roll. At production speeds, no mechanical device has been heretofore known which was capable of detecting gaps in the coating on the metallic foil over a substantial length of the foil. Thus, these gaps were detected visually by an operator of the coating apparatus as the coated foil passed his operating station.
The accuracy of such manual coating continuity detection is limited for several reasons. First, many of the coatings placed on metallic foils are transparent or nearly transparent. Thus, accurate visual inspection is highly difficult. Even when applying coatings that are relatively easy to inspect, distractions of the operator, and the sheer boredom of continuously watching coated foil pass the operator station, can lead to the failure of the operator to detect a substantial coating gap.
It is desirable, therefore, to provide a method and apparatus for mechanically detecting the continuity of coatings on metallic foils which is free from operator judgment.
While major gaps in such coatings cannot be tolerated, complete coverage of even thickness on the foil of the coating material is not always completely necessary in commercial practice. Thus, minor pinholes or thickness variations in the coating may often be tolerated. It is also desirable, therefore, that a mechanized coating continuity detection method and apparatus be capable of differentiating major gaps in the coating requiring corrective attention from minor acceptable gaps, such as pinholes, minor thickness variations and the like, which are not a cause for concern.